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BEIRUT (AP)  A U.N.-brokered peace deal for Syria appeared to collapse Sunday as the government made a new demand that its opponents provide "written guarantees" to lay down their weapons before regime forces withdraw from cities, a call swiftly rejected by the country's main rebel group.

The deal, brokered by U.N. and Arab League envoy Kofi Annan, was due to take effect on Tuesday, paving the way for negotiations to end the country's year-old crisis, which the U.N. estimates has killed 9,000 people.

Annan said last week that Syrian President Bashar Assad had accepted the plan and its call for government forces to pull back from urban centers. But on Sunday Syria's Foreign Ministry spokesman, Jihad Makdessi, placed a new condition — that the opposition agree in writing "to halt violence with all its forms and their readiness to lay down weapons."

The commander of the rebel Free Syrian Army, Riad al-Asaad, said his group was prepared to abide by the Annan agreement, but rejected the government's new unilateral demand.

The FSA does not recognize the regime "and for that reason we will not give guarantees," he told the Associated Press by telephone from his base in neighboring Turkey. He said government forces should return to their bases and remove checkpoints from the streets.

Annan's six-point peace plan calls for government forces to withdraw from population centers by Tuesday, to be followed by a full cease-fire by both sides by 6 a.m. Thursday.

However, with the deadline looming, Syrian forces have stepped up attacks on restive towns in recent days, and activists say scores of civilians have been killed daily.

Annan condemned the government offensive, saying in a statement Sunday that "the present escalation of violence is unacceptable."

He also reminded the government "of the need for full implementation of its commitments," though it was unclear whether his statement was issued before or after Damascus imposed new conditions for complying with the truce.

The cease-fire was meant to pave the way for negotiations between the government and the opposition to end the country's year-old crisis. Street protests against Assad erupted 13 months ago, inspired by the Arab Spring's pro-democracy uprisings in the region, but the revolt has turned violent in the face of a brutal regime crackdown.

Since Assad first agreed to the peace plan, the Syrian opposition and Western leaders have expressed skepticism about his regime's intentions because of broken promises in the past and the recent escalation in attacks on opposition strongholds.

Last year, Assad agreed to an Arab-brokered peace plan similar to Annan's, pledging to work with observers who traveled to Syria on a mission to end the crackdown. But the regime failed to pull out its tanks from towns and cities, saying the country was under attack from the armed groups, and the bloodshed has escalated sharply since the League halted its observer mission on Jan. 28.

Makdessi, the Foreign Ministry spokesman, said Syria will not allow a repeat of what had happened in January, when Assad pulled back his armed forces from cities and their surroundings, only to see rebels flood the areas vacated by government troops.

"Armed terrorist groups used this to rearm its elements and spread its authority on entire districts," Makdessi said. He also alleged that Qatar, Saudi Arabia and Turkey are arming and funding the opposition and demanded guarantees that this will be halted.

Damascus-based activist Maath al-Shami said he is not surprised by the latest government tactic because "whenever the regime pulls out its troops and tanks, people will march to Damascus and topple the regime."

"Frankly speaking, the regime will collapse if checkpoints are removed," he said.

Annan's spokesman, Ahmad Fawzi, refused to comment on the Syrian regime's new conditions.

On Thursday, a U.N. statement raised the possibility of "further steps" if Damascus doesn't implement Annan's six-point peace plan, which Assad agreed to on March 25.

At the Vatican, Pope Benedict XVI implored Damascus to heed international demands to end the bloodshed.

"Particularly in Syria, may there be an end to bloodshed and an immediate commitment to the path of respect, dialogue and reconciliation, as called for by the international community," Benedict said in his traditional Easter speech.

With the deadline looming, Syrian troops pounded restive regions in the north and center of the country Sunday, a day after activists said more than 100 people were killed across Syria.

Some of the heaviest fighting Sunday was taking place in the central city of Homs, where government troops fired mortar rounds at the rebel-held neighborhood of Khaldiyeh, said activist Tarek Badrakhan from Homs.

"Mortar rounds are falling like rain," Badrakhan told the Associated Press via Skype as explosions could be heard in the background. He said troops were attacking on three fronts the rebel-held areas Deir Baalbeh, Khaldiyeh and Old Homs.

"They are giving the regime permission to kill and commit massacres," said Badrakhan referring to the cease-fire. He added that people have not been able to bury the dead in Khaldiyeh because of the shelling. He said that 40 bodies are piled in a room in a makeshift hospital, and that activists are trying to keep the bodies cold by directing a fan toward them.

"We might have to bury them in public gardens," he said.

The Britain-based Syrian Observatory for Human Rights said intense clashes occurred in Homs' Qusour, where defectors were able to kill four government soldiers and damage an armored vehicle.

In the northwest, the Observatory said regime troops were fighting near the town of Jisr al-Shughour. The group also reported clashes between troops and defectors in several towns in the northern province of Aleppo.

Activists also reported raids in the Damascus suburbs of Darya, Douma and Beit Jin.

The Local Coordination Committees said 23 people were killed Sunday, including five in Beit Jin. The Observatory said clashes and government troops shelling killed at least 14 civilians Sunday as well as seven defectors and 12 soldiers.

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